1. The Field of the Invention
This invention relates to modem communication systems that are used by computer equipment to communicate data across public or private telephone lines. More particularly, the invention relates to a line-current protection circuit that protects modem circuitry from being damaged by an excessively high line current that can be supplied via the telephone line in the event of misconnection.
2. Background Art
Increasingly, users of personal computers and related computer equipment rely on the ability to utilize public and private telephone lines to transmit and receive data. This capability dramatically increases the value of the computer, and the productivity of the computer user. As the number of remotely accessible public and private networks, databases, computer-based bulletin boards and related computer services continues to expand, the need for this communication capability will continue to grow.
As is well known, computer equipment, such as personal computers, communicate across telephone lines via a piece of equipment known as a xe2x80x9cmodem.xe2x80x9d The term modem is an acronym derived from the phrase xe2x80x9cmodulator-demodulator,xe2x80x9d which is descriptive of the basic function performed by the modem. A modem permits the personal computer, which is a digital device, to be interfaced with telephone lines by modulating the outgoing digital data so that it is compatible with telephone networks, which are designed to handle analog signals (e.g., speech traffic). Similarly, the modem reverses that process by demodulating incoming analog data from the telephone line so that it is in a digital form that is useable by the computer.
The manner in which telephone equipment, such as a modem, must electrically interface with the public telephone system has also been standardized so as to provide a compatible interface. Generally, modem manufacturers must provide an electrical line interface that moderates all signals or energy being input by the modem into the telephone line. The Federal Communications Commission (FCC) and the various telephone companies require this standard interface. The line interface protects the telephone lines and central telephone systems from being damaged, and thereby insures the integrity and quality of transmissions over the telephone lines.
This line interface circuitry is referred to as the Data Access Arrangement (DAA) circuit. The DAA line interface circuit provides an impedance match between the telephone equipment and the telephone line, and it also isolates and protects the telephone equipment from transient signals and other electrical disturbances that may be present on the telephone line. The DAA line interface circuit also protects the telephone line from any disabling electrical influences that may be generated by the telephone equipment (such as a modem). For example, the public telephone system could be damaged if, instead of transmitting frequency signals, the modem were to inadvertently inject DC power into the telephone line. This situation is avoided because the modem must incorporate the FCC mandated DAA line interface circuit, thereby ensuring that such a damaging signal is not inadvertently transmitted onto the public telephone lines.
As already noted, modems are increasingly being used to interconnect computers via public telephone lines. Initially, modems were largely configured as external accessory units, housed in their own cases, and attached to a port-connector located on the personal computer via a cable (usually a RS-232 port and cable). The external modem is then electrically connected to the telephone line via a standard RJ-type connection scheme, as discussed above. Further, an external modem would be equipped with the standard DAA line interface circuitry, as also described above.
External modems are, and will continue to be, widely used. However, with the increased popularity of smaller portable computers (referred to variously as laptop, notebook, sub-notebook or palm-top computers), external modems are less desirable because of their cumbersome size, their need for an additional cable and computer port and their consequent lack of portability. As such, the external modem is simply not conducive to the needs of a portable computer user. In response to this drawback, smaller modems have been developed that are formed as an integral component within the portable computer. This type of internal modem is located within the housing of the portable computer such that it may be directly interfaced with the telephone line via an RJ-11 physical/electrical media connector and associated cable. The internal modem is also necessarily equipped with the required DAA line interface circuitry.
Although such internal modems are more conducive to the portability requirements of a portable computer, they have several drawbacks. For instance, as an integral component, such an internal modem cannot be easily interchanged with other computers. Further, these types of internal modems also take up limited physical space within the portable computer, often at the expense of other computer components, such as an internal disk drive. Further, as computer housings have continued to be downsizedxe2x80x94even to the extent of being hand-heldxe2x80x94internal spatial restrictions have necessitated that modems be even further reduced in size, without giving up functionality.
Such requirements have resulted in the establishment of standards for the internal accessories of the computer. One set of standards applicable to memory cards has been developed by the Personal Computer Memory Card International Association (PCMCIA). This organization is comprised of hundreds of manufacturers of memory cards and related peripheral equipment. The PCMCIA has published specifications setting forth the spatial standard for all memory cards used in down-sized computers as being restricted to a rectangular space approximately 55 millimeters in width, 85 millimeters in length, and 5 millimeters in depth (commonly referred to as the PCMCIA Type II standard).
In keeping with the PCMCIA standards for memory cards, internal modem manufacturers have adopted the same spatial standards for use with their down-sized modem cards. By complying with the standards established by PCMCIA for memory cards, modem card manufacturers have assured themselves of compatibility and spatial conformity with computers utilizing and complying with the PCMCIA specifications.
The constraints imposed by the PCMCIA specification have resulted in the development of xe2x80x9ccredit cardxe2x80x9d sized modem cards. Thus, most of the components formerly housed in an external or integral modem are now contained within a credit-card sized, PCMCIA card. As with any PCMCIA component, a PCMCIA modem card is merely inserted into any computer equipped with a PCMCIA compliant socket. In this way, a PCMCIA modem is interchangeable with other computers, and requires only a minimal amount of physical space.
Although much reduced in size, a PCMCIA modem is interfaced with an external telephone subscriber line in the same manner as an external or internal modem. Thus, a PCMCIA modem can be connected to the telephone line via a standard, RJ-type connector interface and cable. Further, the PCMCIA modem card also includes the standard DAA line interface circuitry previously discussed.
Because the PCMCIA modem card is equipped with a standard RJ-type connector, portable computer users can connect to a telephone line and communicate from almost anywhere in the world. Although this is an important advantage of a PCMCIA modem equipped portable computer, the capability also raises an important problem.
Many offices, hotels, schools and similar buildings are wired such that the telephone equipment contained within the building is not connected directly to the public telephone system. Rather, buildings are increasingly equipped with xe2x80x9cPrivate Branch Exchangesxe2x80x9d (PBX). A PBX is an automatic switching system that is used to interconnect terminal equipment, such as telephone sets, within a building or campus of buildings. When communication is required outside of the building, the PBX will act as a gateway to the public telephone network.
Typically, telephone equipment is connected to a PBX installation with a standard RJ-type connector, as described above. However, although the physical connection between the PBX and the telephone equipment is the same, some PBX equipment utilizes the electrical wires, or leads, contained within the RJ-11 connection in a manner different from the public telephone network. For example, some PBX systems utilize the two center leads of an RJ-11 connector, commonly referred to as the xe2x80x9ctipxe2x80x9d and the xe2x80x9cringxe2x80x9d leads, in a manner that is significantly different from a public telephone company. More specifically, these PBXs utilize the tip and ring leads to supply power to the specifically designed PBX telephone sets. This power is usually supplied as a differential DC voltagexe2x80x94typically in the range of 12 to 90 voltsxe2x80x94that is applied across the tip and ring leads. However, no resistance is supplied to limit the current supplied. In contrast, the public telephone company places approximately 48 volts across the tip and ring leads, but the current is limited by a resistance of 400 Ohms to 1750 Ohms placed in series with the 48 volt source (thereby resulting a line-current that is usually less than around 120 mA.).
Thus, problems can arise when a piece of telephone equipment, such as a PCMCIA modem, is plugged into a RJ-type telephone jack connected to a PBX. If the PBX utilizes the tip and ring leads in the manner described above, the higher supply voltage of the PBX is applied across the DAA line interface circuitry. Since the resulting current drawn by the DAA circuitry is not limited, and thereby excessively high (i.e., greater than the maximum acceptable level of approximately 120 mA.), the DAA circuitry can be destroyed, and the PCMCIA modem thereby rendered inoperative. This problem is exacerbated by the fact that a portable computer user is usually unaware as to whether a particular RJ-11 (or similar RJ-type connector) telephone jack is connected to such a PBX. Consequently, the user may inadvertently plug the PCMCIA modem into such a xe2x80x9cstandard lookingxe2x80x9d jack, and thereby render the modem inoperative.
Solutions to this particular problem have heretofore not been entirely satisfactory. One solution is to protect the DAA line interface circuitry with a fuse type device. If the PCMCIA modem is taken xe2x80x9coff-hookxe2x80x9d while connected to a PBX that supplies an excessively high current, the fuse or fusible resistor will open, and thereby protect the DAA line interface circuitry. However, the fuse must then be physically replaced before the modem is again operative. Although this approach may work in an external type modem, wherein the fuse is relatively easy to access and replace, it is not practical in a PCMCIA modem card, wherein the electrical components are miniaturized and disposed within a sealed enclosure. Typically, the PCMCIA card must be returned to the manufacturer to have the fuse replacedxe2x80x94a costly and time consuming process.
Further, as is well known, the reaction time of a fuse is relatively slow and often unpredictable. Since fuses are generally slow to react to an excessive current, the DAA line interface circuitry must utilize components that are rated higher (and are thus physically larger) to compensate for the additional wattage consumed before the fuse reacts. This is not practical in a PCMCIA card environment, wherein physical space is extremely limited.
Finally, as noted above, the portable computer user is often unaware that a particular telephone jack is connected to a PBX system that could cause damage to the PCMCIA modem""s DAA circuitry. Thus, even if a PCMCIA modem card is equipped with a fuse, and the modem is inadvertently connected to such a PBX, the modem will no longer be operable because the fuse remains open. The user typically will not know why the PCMCIA modem card has failed and is no longer operative. Diagnosis of the problem is very difficultxe2x80x94again resulting in a costly and a time-consuming process.
Similar xe2x80x9cfusexe2x80x9d like devices have the same drawbacks. For instance, circuit breaking or current limiting devices are also generally slow to react. Further, these types of devices are relatively large, and again cannot physically fit within the limited space of a PCMCIA card.
A further problem arises when a multiplicity of hold current standards are employed. For example, it is known that the United States embraced an xe2x80x9coff-hookxe2x80x9d or hold current profile having a linear voltage and current outline with respect to the voltage and current presented to the modem. Such profile represents boundaries within which corresponding equipment should operate in order to properly signal to the central office on the telephone line that the modem or other device is in an off-hook state. While this linear hold current standard has been adopted by other countries, it is known that a significant number of countries have adopted various other xe2x80x9coff-hookxe2x80x9d profiles that take on various other voltage and current outlines. Most notably, and other prevailing standard that is commonly employed is known as the TBR21 standard. This standard exhibits a non-linear voltage and current profile. However, this non-linear profile more closely approximates the linear profile voltage and current specifications than the PBX specifications. Therefore, there exists a need to be able to accommodate a particular hold-current modem to which the modem has been previously configured while enabling the evaluation of the telephone line and its compatibility with the modem to which it has been connected. It should be reiterated that the evaluation all the voltage and current on the telephone line as presented to the modem or other device must be performed in a very timely manner in order to preclude, or at least minimize, damage to either the modem or other device is connected to the telephone line.
Therefore, there remains a need for a line-current protection circuit that will insure that the DAA line interface circuitry is not damaged when inadvertently subjected to an excessively high current.
The present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art not heretofore fully or completely solved by line current protection schemes used in connection with modem cards. However, it is not intended that the apparatus of the present invention will necessarily be limited solely to modem cards, since it may also find useful application with other devices requiring protection from exposure to excessive line-current conditions. Thus, it is an overall object of the present invention to provide a line-current protection circuit that provides for cost effective and reliable protection of circuitry from an excessive line current condition.
Another object of the present invention is to provide a line-current protection circuit that utilizes electrical components that physically can be used within the limited space of a miniaturized or mobile architecture such as a PCMCIA-architecture card.
Yet another object of the present invention is to provide a line-current protection circuit that is capable of detecting and responding to an excessive line current condition fast enough so as to insure that standard DAA line interface components used within a PCMCIA modem card are not damaged.
Another object of the present invention is to provide a line-current protection circuit that is capable of detecting and not responding to a transient signal present on the telephone line.
Still another object of the present invention is to provide a line-current protection circuit which is automatically reset after the excessive line current condition is removed, so that there is no need to physically remove or otherwise replace circuit components.
A further object of the present invention is to provide a line current protection circuit capable of indicating the presence of an excessive line current condition allowing the PCMCIA modem user to be aware of the condition and the reason for the modem""s inoperability.
A yet further object of the invention is to provide a line current protection circuit capable of operating in a plurality of hold-current modes and detecting when a telephone line, having characteristics outside of the parameters of the specific modes, is connected to a modem.
Additional objects and advantages of the present invention will become more fully apparent from the following detailed description taken in conjunction with the drawings and claims, or may be learned by the practice of the invention.
Briefly summarized, the foregoing and other objects are achieved with a novel line-current protection circuit. The protection circuit is particularly advantageous in that it may be used within small architecture modem configurations such as a PCMCIA-architecture modem card. The protection circuit is oriented for placement in series with the tip lead of a telephone subscriber loop once the PCMCIA modem is plugged into the telephone subscriber loop via a standard RJ-type, or similar, connection. When connected to the telephone line in this manner, a differential DC voltage is presented to the DAA line interface circuitry across the tip and the ring leads of the telephone line. The line protection circuit configures the DAA to operate in a linear mode for purposes of testing for the presence of excess current and voltage which could damage the DAA. If an excessive line current is presented to the DAA line interface circuitry, that condition will be detected by the protection circuit, which will then respond by automatically disconnecting the DAA circuitry from between the tip and ring leads. This automatic disconnection occurs rapidly, before any damage is caused to the DAA circuit electrical components. Further, the connection will not be permitted to be reestablished until such time as the excessive line current condition is removed. If excessive line current is not present, then the DAA is configured to operate in the mode as specified either by the user or manufacturer as originally configured.
The line-current protection circuit is comprised of a configurable hold circuit means for configuring the line interface circuit to operate between at least a first linear mode and a second nonlinear mode. The configurable hold circuit means is controlled by a controller means which is capable of reconfiguring the configurable hold circuit means and activating/deactivating a relay means, which is connected between the DAA interface circuitry and the tip lead of the telephone line, and which selectively provides for either an open or a closed connection between the DAA circuitry and the tip and ring leads of the telephone line. The protection circuit is further comprised of a sensing or detection means, which is capable of detecting the magnitude of the line current that is presented to the DAA line interface circuitry. The results of the line current evaluation in the sensing means is conveyed to the controller means for evaluation which then causes the relay means to either open or close, depending on the magnitude of the line current. Thus, if the line current exceeds a predetermined maximum, such as approximately 120 mA, the current sensing or detection means will sense that condition and therefore cause control circuit means to direct the relay means to open the circuit connection between tip and ring leads. In this manner, excessive current does not reach the DAA line interface circuitry. When the detected current is not excessive, the controller means reconfigures the configurable hold circuit means to operate in the originally configured or operating mode.
In a preferred embodiment of the protection circuit of the present invention, the detection means is comprised of a current sensing means which is for providing an excessive line current signal when the magnitude of the line current presented to the DAA circuitry exceeds a predetermined magnitude.
Also in the preferred embodiment of the protection circuit of the present invention, the controller circuit means is comprised of executable instructions that perform steps of configuring the configurable hold circuit means into a test mode, which in the preferred embodiment is the linear mode, when the configurable hold circuit means is not already configured to the linear mode; detecting excessive line current from a sensing means; disconnecting the line interface circuit from the telephone line when excessive line current is detected; and when excessive line current is not detected, configuring the configurable hold circuit means into an operating mode, either linear or non-linear, as originally selected by the user or programmed by the manufacturer or other party.
The protection circuit may further include a backup fuse means, also connected between the DAA interface circuitry and the tip lead of the telephone line, which is for opening the connection between the DAA interface circuitry and the tip and ring leads if the line current were to exceed a second predetermined maximum magnitude. This fuse means acts as a backup protection in the event that the components making up the relay means or detector means were to fail.